Resistive-switching memory elements having improved switching characteristics

ABSTRACT

Resistive-switching memory elements having improved switching characteristics are described, including a memory element having a first electrode and a second electrode, a switching layer between the first electrode and the second electrode, the switching layer comprising a first metal oxide having a first bandgap greater than 4 electron volts (eV), the switching layer having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a second metal oxide having a second bandgap greater the first bandgap, the coupling layer having a second thickness that is less than 25 percent of the first thickness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part application of U.S.application Ser. No. 12/114,667, filed on May 2, 2008 and issued as U.S.Pat. No. 8,144,498 on Mar. 27, 2012, which claims priority to U.S.Provisional Application No. 60/928,648, filed May 9, 2007.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor memories. Morespecifically, resistive-switching memory elements having improvedswitching characteristics are described.

BACKGROUND OF THE INVENTION

Non-volatile memories are semiconductor memories that retain theircontents when unpowered. Non-volatile memories are used for storage inelectronic devices such as digital cameras, cellular telephones, andmusic players, as well as in general computer systems, embedded systemsand other electronic devices that require persistent storage.Non-volatile semiconductor memories can take the form of removable andportable memory cards or other memory modules, can be integrated intoother types of circuits or devices, or can take any other desired form.Non-volatile semiconductor memories are becoming more prevalent becauseof their advantages of having small size and persistence, having nomoving parts, and requiring little power to operate.

Flash memory is a common type of non-volatile memory used in a varietyof devices. Flash memory uses an architecture that can result in longaccess, erase, and write times. The operational speeds of electronicdevices and storage demands of users are rapidly increasing. Flashmemory is proving, in many instances, to be inadequate for non-volatilememory needs. Additionally, volatile memories (such as random accessmemory (RAM)) can potentially be replaced by non-volatile memories ifthe speeds of non-volatile memories are increased to meet therequirements for RAM and other applications currently using volatilememories.

Resistive-switching memories are memories that include aresistive-switching material (e.g. a metal oxide) that changes from afirst resistivity to a second resistivity upon the application of a setvoltage, and from the second resistivity back to the first resistivityupon the application of a reset voltage. Existing resistive-switchingmemories have switching characteristics (e.g. set, reset, and formingvoltages, retention) that are unsuitable for some applications.

Thus, what is needed is a resistive-switching memory element withimproved switching characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings:

FIG. 1 illustrates a memory array of resistive switching memoryelements;

FIG. 2 illustrates a memory element including a resistive-switchingmaterial and a select element;

FIGS. 3 and 4 are band diagrams and that illustrate energy levels in amemory element with (FIG. 3) and without (FIG. 4) an interface layer;

FIG. 5 is a graph illustrating the dependency of forming voltage on thepresence of an interface layer;

FIG. 6 illustrates a memory element that shares an electrode with adiode that is used as a select element;

FIG. 7 illustrates a portion of a three-dimensional memory array usingmemory elements described herein;

FIGS. 8A and 8B illustrate the memory element and the creation andmanipulation of oxygen vacancies (defects) within the memory elementusing an interface layer;

FIG. 9 is a logarithm of current (I) versus voltage (V) plot for amemory element;

FIG. 10 is a current (I) versus voltage (V) plot for a memory elementthat demonstrates a resistance state change;

FIGS. 11 and 12 are graphs showing the relationship between thicknessesof a metal oxide layer and resulting set voltages, reset voltages, andon/off current ratios for several materials (metal oxides) used inmemory elements described herein; and

FIGS. 13 and 14 are flowcharts describing processes and for controllinginterface layers.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

According to various embodiments, resistive-switching memory elementsare described herein. The memory elements generally have ametal-insulator-metal (MIM) structure in which at least one insulatinglayer is surrounded by two conductive electrodes. Some embodimentsdescribed herein are memory elements that include electrodes ofdifferent materials (e.g. one electrode is doped silicon and one istitanium nitride) surrounding a switching layer of a higher-bandgapmaterial (e.g. hafnium oxide (HfO₂), bandgap=5.7 eV, thickness ˜20-100Å) and a coupling layer of a material having a bandgap that is greaterthan or approximately equal to that of the switching layer (e.g.zirconium oxide (ZrO₂), bandgap=5.8 eV; or aluminum oxide (Al₂O₃),bandgap=8.4 eV). The coupling layer has a thickness that is less than 25percent the thickness of the switching layer, and memory elementsincluding the coupling layer have exhibited improved switchingcharacteristics (e.g. lower set, reset, and forming voltages, and betterretention).

In other embodiments, a metal-rich metal oxide switching layer andtechniques for forming the metal-rich switching layer are described. Themetal-rich switching layer includes increased numbers of defects (e.g.oxygen vacancies), which can be manipulated to improve switchingcharacteristics. The metal-rich switching layer can be deposited, forexample, by reducing the amount of oxidant that is introduced during anatomic layer deposition (ALD) process. In further embodiments,techniques for removing or controlling the size of an interface layerbetween an electrode and a switching layer deposited thereon aredescribed.

I. Switching Operation

It is believed that the resistive switching of the memory elementsdescribed herein is caused by defects in a metal oxide switching layerof the memory element. Generally, defects are formed in or already existin the deposited metal oxide, and existing defects can be enhanced byadditional processes. For example, physical vapor deposition (PVD)processes and atomic layer deposition (ALD) processes deposit layersthat can have some imperfections or flaws. Defects may take the form ofvariances in charge in the structure of the metal oxide: some chargecarriers may be absent from the structure (i.e. vacancies), additionalcharge carriers may be present (i.e. interstitials), or one element cansubstitute for another (i.e. substitutional).

The defects are thought to be electrically active defects (also known astraps) in the bulk of the metal oxide and/or at the interface of themetal oxide and adjoining layers. It is believed that the traps can befilled by the application of a set voltage (to switch from a high to alow resistance state), and emptied by applying a reset voltage (toswitch from the low to the high resistance state). Traps can be inherentin the as-deposited metal oxide (i.e., existing from formation of themetal oxide) or created and/or enhanced by doping and other processes.Doping can be performed using adjacent “doping” layers that interdiffusewith the switching layer, using implantation, or using other techniques.

It is believed that the defects in the switching layer form conductivepercolation paths upon the application of the set voltage. It is furtherbelieved that the percolation paths are removed upon the application ofa reset voltage. For example, a hafnium oxide layer may include oxygenor hafnium vacancies or oxygen or hafnium interstitials that may formtraps which can be used to create percolation paths and alter theconductivity of the hafnium oxide layer.

The switching characteristics of the resistive-switching memory elementscan be tailored by controlling the defects within the metal oxides.Switching characteristics include operating voltages (e.g. set, reset,and forming voltages), operating currents (e.g. on and off currents),and data retention. Defect control is achieved by type, density, energylevel, and spatial distribution within the switching layer. Thesedefects then modulate the current flow based on whether they are filled(passivated/compensated) or unfilled (uncompensated). Adding differentlayers, controlling the formation of the switching layer, implanting,controlling stress, certain thermal treatments are all used to controlthe defect characteristics. In addition, the defects need not be mobile.For example, a coupling layer 212 (see FIG. 2) and an interface layer214 (see FIGS. 2 and 8A-8B) can be used to control locations, depths,densities, and/or type of defects, and techniques can be used to form aswitching layer having an increased number of defects.

Additionally, the metal oxide switching layer can have any phase (e.g.,crystalline and amorphous) or mixtures of multiple phases.Amorphous-phase metal oxides may have increased resistivity, which insome embodiments can lower the operational currents of the device toreduce potential damage to the memory element.

II. Memory Structure

A. Memory Array

FIG. 1 illustrates a memory array 100 of resistive switching memoryelements 102. Memory array 100 may be part of a memory device or otherintegrated circuit. Memory array 100 is an example of potential memoryconfigurations; it is understood that several other configurations arepossible.

Read and write circuitry may be connected to memory elements 102 usingsignal lines 104 and orthogonal signal lines 106. Signal lines such assignal lines 104 and signal lines 106 are sometimes referred to as wordlines and bit lines and are used to read and write data into theelements 102 of array 100. Individual memory elements 102 or groups ofmemory elements 102 can be addressed using appropriate sets of signallines 104 and 106. Memory element 102 may be formed from one or morelayers 108 of materials, as is described in further detail below. Inaddition, the memory elements 102 shown can be stacked in a verticalfashion to make multi-layer 3-D memory arrays (see FIG. 7).

Any suitable read and write circuitry and array layout scheme may beused to construct a non-volatile memory device from resistive switchingmemory elements such as element 102. For example, horizontal andvertical lines 104 and 106 may be connected directly to the terminals ofresistive switching memory elements 102. This is merely illustrative.

If desired, other electrical devices may be associated (i.e., be one ormore of the layers 108) with each memory element 102 (see, e.g., FIG.2). These devices, which are sometimes referred to as select elements,may include, for example, diodes, p-i-n diodes, silicon diodes, siliconp-i-n diodes, transistors, Schottky diodes, etc. Select elements may beconnected in series in any suitable locations in memory element 102.

B. Memory Element

1. MIM Structure

FIG. 2 illustrates a memory element 102 including a resistive-switchingmaterial and a select element (a diode 202). The memory element 102includes a metal-insulator-metal (MIM)-style stack 204 (in someembodiments, one or more of the metal layers can be a conductivesemiconductor material such as doped silicon). The stack 204 includestwo electrodes 206 and 208 and a resistive-switching layer 210 (e.g. aninsulator or metal oxide). The electrodes 206 and 208 can be metals,metal oxides, or metal nitrides (e.g. Pt, Ru, RuO₂, Ir, IrO₂, TiN, W,TaN, MoN, MoOx), or can be doped silicon, for example p- or n-type dopedpolysilicon. The resistive-switching layer 210 can be a metal oxide orother switching material. In some embodiments, the resistive-switchinglayer 210 is a high bandgap (i.e. bandgap greater than four electronvolts (eVs)) material such as HfO₂, Ta₂O₅, Al₂O₃, Y₂O₃, and ZrO₂ (seeFIGS. 11 and 12).

a. Switching Layer

The switching layer 210 can have any desired thickness, but in someembodiments can be between 10 and 100 Å, between 20 and 60 Å, orapproximately 50 Å. The switching layer 210 can be deposited using anydesired technique, but in some embodiments described herein is depositedusing ALD, or a combination of ALD and PVD. In other embodiments, theswitching layer 210 can be deposited using low pressure CVD (LPCVD),plasma enhanced CVD (PECVD), plasma enhanced ALD (PEALD), liquiddeposition processes, and epitaxy processes.

The switching layer 210 additionally can be metal-rich (e.g. HfO_(1.7)vs. HfO₂) such that the elemental composition of the switching layer 210is less than stoichiometric (e.g. less than HfO₂). The switching layer210 can have a deficit of oxygen, which manifests as oxygen vacancydefects. The additional defects can lead to reduced and more predictableswitching and forming voltages of the memory element 102. Techniques fordepositing a metal-rich switching layer 210 are described in FIG. 13.

b. Coupling Layer

The stack 204 can also include a coupling layer 212, which may beanother metal oxide such as ZrO₂ or Al₂O₃. In other embodiments, thecoupling layer 212 can be deposited as a metal layer that will oxidizeupon the deposition of the adjacent electrode 208 or upon annealing. Thecoupling layer 212 can, for example, facilitate switching at theelectrode 208 by creating defects near the electrode 208. The couplinglayer 212 can be thinner than the resistive-switching layer 210, forexample the coupling layer 212 can have a thickness that is less than25% of the thickness of the resistive-switching layer 210, or athickness that is less than 10% of the thickness of theresistive-switching layer 210. For example, the resistive-switchinglayer 210 can be a 20-60 Å layer, and the interface layer 212 can be a5-10 Å layer.

The coupling layer 212 in some embodiments has a bandgap that isapproximately equal to or greater than a bandgap of the switching layer210. The higher bandgap of the coupling layer 212 can help improveretention of the memory element 102 by reducing leakage from theswitching layer 210. Additionally, the coupling layer 212 can createdefects near the electrode 208 (including in near or at the interfacebetween the coupling layer and the switching layer 210 and/or near or atthe interface between the electrode 208 and the coupling layer 212),which can assist in switching. The coupling layer 212 is thin enough toprovide access to defects in the switching layer 210.

c. Interface Layer

The stack 204 further may include an interface layer 214 between theelectrode 206 and the switching layer 210. The interface layer 214 canbe an oxide of the material of the electrode 206 that is formed as aresult of and during the deposition of the switching layer 210, forexample as a result of thermal oxidation during processing. Theinterface layer 214 can, in some embodiments, alter defects in theswitching layer 210 (see, e.g. FIGS. 8A-8B). In other embodiments, itmay be desirable to eliminate the interface layer 214 to reduce formingvoltage or to enable switching. It is believed that in some embodiments,the interface layer 214 can hinder effective electron injection into theswitching layer 210 that enables traps to be filled, which therebyincreases forming voltage or causes excessive potential drop across it,producing high electric fields in the switching layer 210 and preventingswitching. Techniques for controlling the size of or eliminating theinterface layer 214 are described in FIGS. 13 and 14.

FIGS. 3 and 4 are band diagrams 300 and 400 that illustrate energylevels in a memory element with (FIG. 3) and without (FIG. 4) aninterface layer 214. For each of the band diagrams 300 and 400, thereare corresponding electric field diagrams 320 and 340 that illustratethe strength of the electric field within a certain region of the memoryelement 102.

In the band diagram 300, a memory element has a titanium nitrideelectrode 302, a zirconium oxide coupling layer 304, a hafnium oxideswitching layer 306, a silicon oxide interface layer 308, and an n-typepolysilicon electrode 310. In the band diagram 400, a memory element hasa titanium nitride electrode 402, a zirconium oxide coupling layer 404,a hafnium oxide switching layer 406, and an n-type polysilicon electrode408. As is shown in the electric field diagram 320, the electric fieldis reduced by a large amount 322 in the interface layer 314. Increasedswitching voltages may be necessary to overcome the electric fieldreduction in the interface layer 214. If the interface layer 214 isthick enough, the entire electric field may be lost to the interfacelayer 214, which may prevent switching altogether. Alternatively, as isshown in the electric field diagram 420, in the memory element withoutthe interface layer 214 the electric field is reduced evenly 422throughout the memory element 102, including in the switching layer 210,which can reduce switching voltages and lead to more predictableswitching. However, as is described regarding FIGS. 8A and 8B, it may bedesirable to retain a controlled-thickness interface layer 214 toincrease the number of defects in the switching layer 210.

FIG. 5 is a graph 500 illustrating the dependency of forming voltage onthe presence of an interface layer 214. Three sets of memory elementswere prepared:

-   -   A first set of memory elements represented by diamonds 502        includes a titanium nitride electrode 206, a PVD-deposited        hafnium oxide switching layer 210, and a platinum electrode 208        without a coupling layer 212.    -   A second set of memory elements represented by squares 504        includes an n-type polysilicon electrode 206, an ALD-deposited        hafnium oxide switching layer 210, and a titanium nitride        electrode 208 without a coupling layer 212.    -   A third set of memory elements represented by a circle 506        includes an n-type polysilicon electrode 206, a PVD-deposited        hafnium oxide switching layer 210, and a platinum electrode 208        without a coupling layer 212.

The graph 500 shows the median forming voltage of the memory elements asa function of the thickness of the switching layer in the memoryelements. As can be seen, for a switching layer having the samethickness, the elements 502 including PVD hafnium oxide on titaniumnitride have the lowest forming voltage, elements 506 including PVDhafnium oxide on polysilicon have the next lowest forming voltage, andelements 504 having ALD hafnium oxide on polysilicon have the highestforming voltage. It is believed that ALD processes are more likely toform a thicker interface layer 214 at least partly because ofpotentially higher processing temperatures (200° C. or greater versusroom temperature for some instances of PVD), which leads to increasedforming voltages. Additionally, the silicon oxide interface layer 214created on polysilicon electrodes (e.g. the elements 502 and 506) isless conductive than an oxide created on a metal-containing electrodesuch as titanium nitride. Therefore, techniques for reducing and/orcontrolling the interface layer 214, especially for silicon-basedelectrodes, can be used to improve forming voltages.

Although ALD process may be more likely to form thicker interface layers214 and result in memory elements having increased forming voltages, itmay be desirable to use ALD processing over PVD processing for otherreasons (e.g. to form more conformal layers), and FIG. 13 describes aprocess for reducing or eliminating the interface layer 214 using ALDprocessing. Additionally, as is described regarding FIGS. 8A and 8B, itmay be desirable to retain a controlled-thickness interface layer 214(e.g. less than or equal to 10 Å) to increase the number of defects inthe switching layer 210, which can also be formed using the process ofFIG. 13.

If it is desirable to have an interface layer 214, the order ofdeposition of the layers of the MIM stack 204 may be important. Sincethe interface layer 214 is formed during the deposition of the switchinglayer 210, the switching layer 210 can be formed on the electrode thatthe interface layer 214 is to be formed from (e.g. formed on thepolysilicon layer if a silicon oxide interface layer 214 is desired). Asan example, and as is discussed further in FIG. 7, when forming athree-dimensional memory array, it may be necessary to always form thememory element in the same orientation (e.g. one electrode always on thebottom), even when the orientation of other elements is to be reversed.In other embodiments however, the interface layer 214 can be createdwhen the memory element 102 is deposited in reverse order by using apost deposition anneal of the memory element 102.

d. Electrodes

The electrodes 206 and 208 can be different materials. In someembodiments, the electrodes have a work function that differs by between0.1 and 1 electron volt (eV), or by between 0.4 and 0.6 eV, etc. Forexample, the electrode 208 can be TiN, which has a work function of4.5-4.6 eV, while the electrode 206 can be n-type polysilicon, which hasa work function of approximately 4.1-4.15 eV. Other electrode materialsinclude p-type polysilicon (4.9-5.3 eV), tungsten (4.5-4.6 eV), tantalumnitride (4.7-4.8 eV), molybdenum oxide (approximately 5.1 eV),molybdenum nitride (4.0-5.0 eV), iridium (approximately 5.3 eV), iridiumoxide (approximately 4.2 eV), ruthenium (approximately 4.7 eV), andruthenium oxide (approximately 5.0 eV). For some embodiments describedherein, the higher work function electrode receives a positive pulse (asmeasured compared to a common reference potential) during a resetoperation, although other configurations are possible. In otherembodiments, the higher work function electrode receives a negativepulse during a reset operation. In some embodiments, the memory elements102 use bipolar switching where the set and reset voltages have oppositepolarities relative to a common electrical reference, and in someembodiments the memory elements 102 use unipolar switching where the setand reset voltages have the same polarity.

2. Select Elements

The diode 202 is a select element that can be used to select a memoryelement for access from amongst several memory elements such as theseveral memory elements 102 of the memory array 100 (see FIG. 1). Thediode 202 controls the flow of current so that current only flows oneway through the memory elements 102.

The diode 202 may include two or more layers of semiconductor material.A diode is generally a p-n junction, and doped silicon layers 216 and218 can form the p-n junction. For example, doped silicon layer 216 canbe a p-type layer and doped silicon layer 218 can be an n-type layer, sothat a node 220 of the diode 202 is an anode and is connected to thefirst electrode 206. In this example, a node 222 of the diode 202 is acathode and is connected to the signal line 106, which may be, forexample, a bit line or word line, or connected to a bit line or wordline. The nodes 220 and 222 are not necessarily physical features in thememory element 102, for example the electrode 206 may be in directcontact with the doped silicon layer 216. In other embodiments, one ormore additional layers such as a low resistivity film are added betweenthe electrode 206 and the doped silicon layer 216.

In some embodiments, doped silicon layer 216 is an n-type layer anddoped silicon layer 218 is a p-type layer, and the node 220 is a cathodeof the diode 202 and the node 222 is an anode of the diode 202. Anoptional insulating layer 224 can be between the doped silicon layers216 and 218 to create a p-i-n or n-i-p diode 202. In some embodimentsthe insulating layer 224 and one of the doped silicon layers 216 and 218are formed from the same layer. For example, a silicon layer can bedeposited, and a portion of the layer can be doped to form the dopedsilicon layer 216 or 218. The remaining portion of the layer is then theinsulating layer 224.

In other embodiments, one electrode of the memory element 102 can bedoped silicon (e.g. p-type or n-type polysilicon), which can also act asa portion of the diode 202. FIG. 6 illustrates a memory element 102 thatshares an electrode with a diode 202 that is used as a select element.Since the diode 202 is made up of two layers of doped silicon, and sincea layer of doped silicon can be used as an electrode of the memoryelement 202, a single layer of doped silicon (e.g. a layer of n-typepolysilicon) can serve as an electrode of the memory element 102 and asa layer of the diode 202. By sharing a doped silicon layer between thediode 202 and the memory element 102, two layers, one doped siliconlayer and a coupling layer between the diode 202 and the memory element102, and their associated processing steps, can be eliminated.

3. Switching Polarity

A signal line (e.g. the signal line 104) is connected to the “second”electrode 208, and the signal line is configured to provide switchingvoltages to the second electrode 208. In some embodiments, the secondelectrode 208 has a higher work function than the first electrode 206,and the signal line 104 is configured to provide a negative set voltagerelative to a common electrical reference, and a positive reset voltagerelative to the common electrical reference. The embodiments may includethose using a lower work function first electrode 206 (e.g. titaniumnitride) and a higher work function second electrode such as platinum orruthenium. For example, the common electrical reference may be ground(i.e. 0V), the set voltage would then be a negative voltage (e.g. −2V),and the reset voltage would be a positive voltage (e.g. 2V). The commonelectrical reference can be any voltage, however, such as +2V or −2V.

In other embodiments, the second electrode 208 also has a higher workfunction than the first electrode 206, and the signal line 104 isconfigured to provide a positive set voltage and a negative resetvoltage relative to a common electrical reference. For example, in amemory element having a doped silicon first electrode 206 (e.g. n-typepolysilicon) and a higher work function second electrode 208 (e.g.titanium nitride), the reset voltage can be negative at the secondelectrode 208.

Generally, in some embodiments, one switching voltage (e.g. the resetvoltage) of the memory element can have a first polarity (e.g. apositive polarity) relative to the common electrical reference, and theother switching voltage (e.g. the set voltage) can have a negativepolarity relative to the common electrical reference so that the memoryelement uses bipolar switching. In other embodiments, the switchingvoltages have the same polarity relative to a common reference and arereferred to as unipolar switching. Additionally, the switching voltagescan be voltage pulses (e.g. square wave pulses) having a limitedduration, for example less than 1 ms, less than 50 μs, less than 1 μs,less than 50 ns, etc.

4. Polarity of Forming Voltage

Lower operating voltages are desirable for resistive switching memoryelements to protect associated devices (e.g. diodes) in the memoryarray. Forming voltage is often the highest magnitude operating voltage,and reduction of the forming voltage is therefore an important goal toimprove device operation and reliability. Forming voltage polarity hasbeen shown to affect forming voltage magnitude in some embodiments.

In one example, memory elements were prepared with a higher workfunction electrode connected to ground and a lower work functionelectrode receiving the forming voltage pulse. A first example includedan n-type polysilicon electrode, a hafnium oxide switching layer and atitanium nitride electrode. In this example, the titanium nitrideelectrode (i.e. the higher work function electrode) was grounded andpositive and negative forming voltage pulses were applied to the n-typepolysilicon electrode. The negative pulses had a median forming voltageof approximately −8V, while the positive pulses had a median formingvoltage of approximately +13V. In a second example, a memory element wasprepared having a titanium nitride electrode, a hafnium oxide switchinglayer, and a platinum electrode. The higher work function electrode(here, the platinum electrode) was grounded, and the lower work functionelectrode (the titanium nitride electrode) received forming voltagepulses. In this example, the median forming voltage of negative pulseswas −4.4V, while the median forming voltage using positive pulses was6.4V.

The examples above demonstrate that a negative forming voltage appliedat the lower-work function electrode can reduce the magnitude of formingvoltage in some embodiments. It is believed that electron injection fromthe lower-work function electrode can reduce the magnitude of a negativepolarity forming voltage compared to a positive polarity formingvoltage.

Additionally, it has been demonstrated for some embodiments that thegreater the difference between the work function of the electrodes in amemory element, the smaller the magnitude of the forming voltage. Forexample, in a memory element with an n-type polysilicon electrode, ahafnium oxide switching layer and a titanium nitride electrode, thedifference in work function is approximately 0.5 eV and a median formingvoltage is −7V. Another memory element having an n-type polysiliconelectrode, a hafnium oxide switching layer, and a platinum electrode hasa work function difference of 1.6 eV and a median forming voltage of−5.5V. Therefore, in some embodiments it may be desirable to increasethe work function difference between the electrodes to reduce theforming voltage, although it may not be necessary to increase the workfunction difference to 1.6 eV as in the example.

5. Other Characteristics

It may be desirable to have a low-leakage material as theresistive-switching layer 210 in order to aid memory retention. Forexample, the layer 210 may be a material that has a leakage currentdensity less than 40 amps per square centimeter (A/cm²) measured at 0.5volts (V) per twenty angstroms of the thickness of the metal oxide in anoff state (e.g. a high resistance state) of the memory element.

6. 3-D Memory Structure

FIG. 7 illustrates a portion of a three-dimensional memory 700 arrayusing memory elements 102 described herein. The array 700 includes twoword lines 702 a and 702 b, and a shared bit line 704. Two MIM stacks204 a and 204 b and diodes 202 a and 204 b are shown in the array 700; amemory cell 706 a includes an MIM stack 204 a and a diode 202 a, and amemory cell 706 b includes an MIM stack 204 b and a diode 202 b.

The memory array 700 is configured so that the two memory cells 706 aand 706 b can use the same shared bit line 704. As shown here, the MIMstacks 204 a and 204 b both have their individual layers (i.e.electrodes 206 and 208 and switching layer 210) built in the same order.In other words, for both MIM stacks 204 a and 204 b, the electrode 206is formed first, the switching layer 210 is formed on top of theelectrode 206, and the electrode 208 is formed on top of the switchinglayer 210. As mentioned above, the order of deposition of the layers ofthe MIM stacks 204 may need to be the same in order to create aninterface layer 214. However, in some embodiments the order ofdeposition can be reversed and the interface layer 214 created as aresult of subsequent processes such as electrode deposition orannealing.

The diodes 202 a and 202 b, on the other hand, are minors of each other.In other words the diode 202 a has the layer 216 on the bottom, and thediode 202 b has the layer 218 on the bottom. For example, the layer 216may be the n-type layer and the layer 218 may be the p-type layer. Usingthis configuration, the diodes 202 a and 202 b are biased in oppositedirections, which allows the memory cells 706 to both use the sameshared bit line 704. As is shown in circuit diagrams 708 a and 708 b,the diodes can have any desired orientation, and the orientation maydiffer based on the configuration of the three-dimensional memory array.

7. Interface Layer and Oxygen Vacancies

FIGS. 8A and 8B illustrate the memory element 102 and the creation andmanipulation of oxygen vacancies (defects) within the memory element 102using an interface layer 214. The interface layer 214 is an oxide layerthat can be created during the processing of other layers in the memoryelement 102. For example, the deposition of the switching layer 210 mayinclude processing at a temperature (e.g. 200° C. or greater) to createthe interface layer 214. If, for example, the electrode 206 is dopedsilicon (e.g. polysilicon), the deposition of the switching layer 210(using, for example, PVD or ALD) may include temperatures that cancreate a silicon oxide interface layer 214. The interface layer 214 canbe eliminated in some embodiments, but in other embodiments, theinterface layer 214 can be retained to improve retention of theswitching layer 210 by improving leakage characteristics and to modulatedefects (e.g. oxygen vacancies) in the switching layer 210. In someembodiments where the interface layer 214 is retained, the interfacelayer 214 may be relatively thin (e.g. less than or equal to 10 Å) tomake the defects in the switching layer 210 visible to the electrode 206(i.e. the interface layer 214 provides access to the defects of theswitching layer 210) and to reduce the effect of the interface layer 214on switching voltages.

In one example, the bottom electrode 206 is polysilicon. Silicon,particularly, is known for attracting oxygen when heated and can drawoxygen from the metal oxide switching layer 210, leaving oxygenvacancies 802 in the switching layer 210 nearby creating a metal-richmetal oxide switching layer. Without being bound by theory, these oxygenvacancies 802 can serve as traps which modulate the current flow withthe application of programming voltages to fill and empty such traps.The oxygen vacancies 802 need not be mobile.

A thin or zero interlayer thickness interface layer 214 can be used tomodulate the density of oxygen vacancies 802 in the switching layer 210.For example, a thinner interface layer 214 (e.g. 5 Å vs. 10 Å) canincrease the oxygen vacancy 802 density. Additionally, the thickness ofthe switching layer 210 can be optimized such that traps (e.g. oxygenvacancies 802) are more spatially equalized throughout the switchinglayer 210. For example, FIG. 8A shows a thicker switching layer 210,which has oxygen vacancies 802 concentrated near the interface layer214, while FIG. 8B shows a thinner switching layer 210 that has a moreeven distribution of oxygen vacancies 802. For example, in two memoryelements using the same materials, the switching layer 210 of FIG. 8Amay be 50 Å while the thickness of the switching layer 210 in FIG. 8B is25 Å. The distribution of oxygen vacancies 802 within the switchinglayer 210 can depend on several factors, including the materials used,the thickness of the interface layer 214, the processes used (e.g.temperatures of anneals used), etc. FIGS. 8A and 8B are only twoexamples of oxygen vacancy distribution, and it is understood thatvarious other configurations are possible.

III. Memory Operation

During a read operation, the state of a memory element 102 can be sensedby applying a sensing voltage (i.e., a “read” voltage V_(READ)) to anappropriate set of signal lines 104 and 106. Depending on its history, amemory element that is addressed in this way may be in either a highresistance state or a low resistance state. The resistance of the memoryelement therefore determines what digital data is being stored by thememory element. If the memory element has a low resistance, for example,the memory element may be said to contain a logic one (i.e., a “1” bit).If, on the other hand, the memory element has a high resistance, thememory element may be said to contain a logic zero (i.e., a “0” bit).During a write operation, the state of a memory element can be changedby application of suitable write signals to an appropriate set of signallines 104 and 106.

FIG. 9 is a logarithm of current (I) versus voltage (V) plot 900 for amemory element 102. FIG. 9 illustrates the set and reset operations tochange the contents of the memory element 102. Initially, memory element102 may be in a high resistance state (“HRS”, e.g., storing a logiczero). In this state, the current versus voltage characteristic ofmemory element 102 is represented by solid line HRS 902. The highresistance state of memory element 102 can be sensed by read and writecircuitry using signal lines 104 and 106. For example, read and writecircuitry may apply a read voltage V_(READ) to memory element 102 andcan sense the resulting “off” current I_(OFF) that flows through memoryelement 102. When it is desired to store a logic one in memory element102, memory element 102 can be placed into its low-resistance state.This may be accomplished by using read and write circuitry to apply aset voltage V_(SET) across signal lines 104 and 106. Applying V_(SET) tomemory element 102 causes memory element 102 to switch to its lowresistance state, as indicated by dashed line 906. In this region, thememory element 102 is changed so that, following removal of the setvoltage V_(SET), memory element 102 is characterized by low resistancecurve LRS 904. As is described further below, the change in theresistive state of memory element 102 may be because of the filling oftraps (i.e., a may be “trap-mediated”) in a metal oxide material.V_(SET) and V_(RESET) can be generally referred to as “switchingvoltages.”

The low resistance state of memory element 102 can be sensed using readand write circuitry. When a read voltage V_(READ) is applied toresistive switching memory element 102, read and write circuitry willsense the relatively high “on” current value I_(ON), indicating thatmemory element 102 is in its low resistance state. When it is desired tostore a logic zero in memory element 102, the memory element can onceagain be placed in its high resistance state by applying a reset voltageV_(RESET) to memory element 102. When read and write circuitry appliesV_(RESET) to memory element 102, memory element 102 enters its highresistance state HRS, as indicated by dashed line 908. When the resetvoltage V_(RESET) is removed from memory element 102, memory element 102will once again be characterized by high resistance line HRS 904.Voltage pulses can be used in the programming of the memory element 102.For example, a 1 ms, 10 μs, 5 μs, 500 ns, etc. square pulse can be usedto switch the memory element 102; in some embodiments, it may bedesirable to adjust the length of the pulse depending on the amount oftime needed to switch the memory element 102.

A forming voltage V_(FORM) is a voltage applied to the memory element102 to ready the memory element 102 for use. Some memory elementsdescribed herein may need a forming event that includes the applicationof a voltage greater than or equal to the set voltage or reset voltage.Once the memory element 102 initially switches the set and resetvoltages can be used to change the resistance state of the memoryelement 102.

The bistable resistance of resistive switching memory element 102 makesmemory element 102 suitable for storing digital data. Because no changestake place in the stored data in the absence of application of thevoltages V_(SET) and V_(RESET), memory formed from elements such aselement 102 is non-volatile.

FIG. 10 is a current (I) versus voltage (V) plot 1000 for a memoryelement 102 that demonstrates a resistance state change. The plot 1000shows a voltage ramp applied to the memory element 102 along the x-axisand the resulting current along a y-axis. The line 1002 represents theresponse of an Ohmic material when the ramped voltage is applied. AnOhmic response is undesirable, since there is no discrete voltage atwhich the set or reset occurs.

Generally, a more abrupt response like graph 1004 is desired. The graph1004 begins with an Ohmic response 1004 a, and then curves sharplyupward 1004 b. The graph 1004 may represent a set operation, where thememory element 102 switches from the HRS 902 to the LRS 904.

Without being bound by theory, non-metallic percolation paths are formedduring a set operation and broken during a reset operation. For example,during a set operation, the memory element 102 switches to a lowresistance state. The percolation paths that are formed by filling trapsincrease the conductivity of the metal oxide, thereby reducing (i.e.,changing) the resistivity. The voltage represented by 404 b is the setvoltage. At the set voltage, the traps are filled and there is a largejump in current as the resistivity of the metal oxide decreases.

IV. Materials

A variety of metal oxides can be used for the switching layer 210 of thememory elements 102 described herein. In some embodiments, the memoryelements 102 exhibit bulk-switching properties and are scalable. Inother words, it is believed that defects are distributed throughout thebulk of the switching layer 210, and that the switching voltages (i.e.V_(SET) and V_(RESET)) increase or decrease with increases or decreasesin thickness of the metal oxide. In other embodiments, the memoryelements 102 exhibit interface-mediated switching activity. Otherembodiments may exhibit a combination of bulk- and interface-mediatedswitching properties, which may be scalable while still exhibitingdefect activity at layer interfaces.

A. Higher-Bandgap Materials for Switching Layer

FIGS. 11 and 12 are graphs showing the relationship between thicknessesof a metal oxide layer and resulting set voltages, reset voltages, andon/off current ratios for several materials (metal oxides) used inmemory elements described herein. These graphs illustrate thecharacteristics of a memory element that includes two electrodes and asingle layer of metal oxide disposed in between (i.e. without a couplinglayer 212) and indicate that certain materials exhibit bulk-switchingproperties. As can be seen in FIG. 11, for memory elements includinghafnium oxide 1102, aluminum oxide 1104, or tantalum oxide 1106, setvoltage increases with (i.e. is dependent on) thickness, and in someembodiments and for these materials the set voltage is at least one volt(V) per one hundred angstroms (Å) of the thickness of a metal oxidelayer in the memory element. In some embodiments, an increase in thethickness of the metal oxide layer of 100 Å increases the set voltage byat least 1V. Similarly, as shown in FIG. 12, reset voltage for hafniumoxide 1202, aluminum oxide 1204, or tantalum oxide 1206 also depends onthickness. These data therefore support a bulk-controlled set/resetmechanism for these materials, since a linear relationship indicates theformation of percolation paths throughout the bulk of the metal oxide.In other words, for a thicker material, more voltage is needed to fillthe traps.

Hafnium oxide (HfO₂, 5.7 electron volts (eV)), aluminum oxide (Al₂O₃,8.4 eV) and tantalum oxide (Ta₂O₅, 4.6 eV) all have a bandgap greaterthan 4 eV, while titanium oxide (TiO₂, 3.0 eV) and niobium oxide (Nb₂O₅,3.4 eV) have bandgaps less than 4 eV. Other higher bandgap metal oxidesthat can be used with various embodiments described herein includeyttrium oxide (Y₂O₃, 6.0 eV) and zirconium oxide (ZrO₂, 5.8 eV) (alsosee Table 1). As shown in FIGS. 11 and 12, set voltages for titaniumoxide 1108 and niobium oxide 1110 and reset voltages for titanium oxide1208 and niobium oxide 1210 do not increase with thickness. Therefore, ahigher bandgap (i.e., bandgap greater than 4 eV) metal oxide exhibitsbulk mediated switching and scalable set and reset voltages. Table 1summarizes the higher-bandgap materials that can be used for switchinglayers 210.

TABLE 1 Material Bandgap HfO₂ 5.7 eV Al₂O₃ 8.4 eV Ta₂O₅ 4.6 eV Y₂O₃ 6.0eV ZrO₂ 5.8 eVB. Coupling Layer Materials

The coupling layer 212 can be a metal oxide material that is chosen tocomplement the material of the switching layer 210. For example, thecoupling layer 212 may be chosen to complement a bandgap of theswitching layer 210. In some embodiments, the coupling layer 212 has abandgap that is approximately equal to or greater than the bandgap ofthe switching layer 210. Without being bound by theory, this can improveretention of the memory element 102 by improving leakagecharacteristics. As is shown in the band diagrams 300 and 400 in FIGS. 3and 4, a zirconium oxide coupling layer 212 has a bandgap that isgreater than the bandgap of the switching layer 210. The higher bandgapcoupling layer 212 can promote retention by reducing leakage from theswitching layer 210 into the electrode 208.

In other examples, the coupling layer 212 can have a lower bandgap thatthe switching layer 210 (for example, the coupling layer 212 can betitanium oxide (bandgap=3.5 eV)), but with some systems this may resultin higher operational voltages and lower yields.

In some examples, a switching layer 210 is hafnium oxide (bandgap=5.7eV) and has a first thickness (e.g. 20-100 Å). The coupling layer canthen be either zirconium oxide (ZrO₂, bandgap=5.8 eV) or aluminum oxide(Al₂O₃, bandgap=8.7 eV) having a second thickness that is less than 25percent of the first thickness. For example, the coupling layer can bebetween 1 and 10 Å thick, or 5 Å or 8 Å.

It has been shown (see Table 3) for some materials systems thatswitching performance is better when the coupling material is in adiscrete coupling layer 212 rather than dispersed throughout theswitching layer 210 (e.g. using a hafnium oxide switching layer 210 andan aluminum oxide coupling layer 212 rather than a HfAlOx layer). It isbelieved that the defects created at the interface between the couplinglayer 212 and the switching layer 210 can improve switchingcharacteristics.

In some embodiments, the coupling layer 212 can be used to dope into theswitching layer 210. The doping can be either aliovalent or isovalent.In aliovalent doping, the doping species has a different valency thanthat of the layer being doped. For example, the switching layer 210 canbe hafnium oxide and the coupling layer 212 can be aluminum oxide. Atypical defect species of hafnium oxide is Hf⁺⁴, and a typical defectspecies of aluminum oxide is Al⁺³. Al⁺³ ions displace Hf⁺⁴ ions in thehafnium oxide layer, thereby creating defects and traps. In someembodiments, the doping is isovalent, and a coupling layer 212 (e.g.,zirconium oxide) may have a metal having the same most common oxidationstate (e.g., +4) as a metal of the switching layer 210. In these cases,aliovalent doping may still occur when other species having differentoxidation states (e.g., Zr⁺³) diffuse into the switching layer 210.

C. Electrodes

Various electrodes can be used for the memory elements 102. Someembodiments describe memory elements 102 that use electrodes 206 and 208that are made of different materials. For example, the electrodes 206and 208 can have materials that are chosen to have different workfunctions (e.g. between 0.4 eV and 0.6 eV different, or between 0.1 eVand 1.0 eV different), which it is believed may facilitate bipolarswitching in some systems.

Materials that can be used for electrodes 206 and 208 include dopedsilicon (e.g. p-type or n-type silicon), titanium nitride, tantalumnitride, tungsten, tungsten nitride, molybdenum nitride, molybdenumoxide, platinum, ruthenium, ruthenium oxide, iridium, and iridium oxide.Electrode “pairs” may include n-type polysilicon and titanium nitride;titanium nitride, tungsten nitride, or tantalum nitride and platinum,ruthenium, ruthenium oxide, iridium, iridium oxide, molybdenum nitride,or molybdenum oxide, although other pairings are possible. Otherelectrodes include metal silicides (see FIG. 14) and electrolesslydeposited electrodes (e.g. electroless nickel). These electrodes can beused to eliminate the silicon oxide interface layer 214 and thereforereduce forming voltages.

In some embodiments, the electrodes 206 and 208 can be chosen to dopeisovalently into the switching layer 210. In other words, at least oneof the electrodes 206 and 208 has a most common oxidation state orvalency that is the same as the most common oxidation state or valencyof the switching layer 210. In some memory elements 102, it is believedthat isovalent doping can create deep traps in the switching layer 210.For example, the electrode 206 can be doped silicon (+4 valency) and theswitching layer 210 can be hafnium (+4 valency) oxide. In otherembodiments, the electrodes 206 or 208 can contain titanium nitride(titanium has +4 valency), platinum (+4 valency), etc. The siliconisovalently dopes into the hafnium oxide, creating deep traps that canbe used to create a greater resistance change and a higher on/offcurrent ratio. Aliovalent doping may in some instances create donors andacceptors, which are shallow traps and may result in aresistive-changing memory that does not exhibit as great a difference inresistance states.

D. Material Systems

Table 2 includes a list of possible materials systems for memoryelements 102 described herein. Although certain combinations aredescribed in Table 2, various other configurations are possible withinthe bounds of the memory elements 102 described herein. For example,other electrode materials (e.g. molybdenum nitride or molybdenumnitride) or switching materials can be used.

TABLE 2 Electrode Interface Switching Coupling Electrode 206 Layer 214Layer 210 Layer 212 208 1 n-type 0-10Å SiOx HfOx 30-100Å AlOx 1-10Å, TiNpolysilicon or ~50Å or ~5Å or ~8Å 2 n-type 0-10Å SiOx HfOx 30-100Å ZrOx1-10Å, TiN polysilicon or ~50Å or ~5Å or ~8Å 3 Ni, Co, Ti, Minimal HfOx30-100Å Optional TiN Pd, Pt or ~50Å (AlOx, ZrOx, silicide TiOx) 4 n-type0-10Å SiOx AlOx, TaOx, Optional TiN polysilicon YOx, ZrOx 30- (AlOx,ZrOx, 100Å, or ~50Å TiOx) 5 TiN 0-10Å HfOx, AlOx, Optional Pt, Ru, TaOx,YOx, (AlOx, ZrOx, RuOx, ZrOx 30-100Å, TiOx) Ir, IrOx or ~50Å 6 ELD NiMinimal HfOx, AlOx, Optional Pt, Ru, TaOx, YOx, (AlOx, ZrOx, RuOx, ZrOx30-100Å, TiOx) Ir, IrOx, or ~50Å TiN

V. Processing

FIGS. 13 and 14 are flowcharts describing processes 1300 and 1400 forcontrolled deposition of interface layers 214. The process 1300describes the deposition of a switching layer 210 using an ALD processthat reduces the amount of oxygen introduced to create a metal-richswitching layer 210 and increase the amount of defects in the switchinglayer 210. Additionally, the process 1300 can be used to tailor the sizeof the interface layer 214 by selecting processing parameters to obtaina desired thickness of the interface layer 214. The process 1400describes the deposition of a silicide electrode 206 that significantlyreduces or eliminates the interface layer 214.

Atomic layer deposition (ALD) is a process used to deposit conformallayers with atomic scale thickness control during various semiconductorprocessing operations. For depositing a metal oxide, ALD is a multi-stepself-limiting process that includes the use of two reagents: a metalprecursor and an oxygen source (e.g. an oxidant). Generally, a firstreagent is introduced into a processing chamber containing a substrateand adsorbs on the surface of the substrate. Excess first reagent ispurged and/or pumped away. A second reagent is then introduced into thechamber and reacts with the adsorbed layer to form a deposited layer viaa deposition reaction. The deposition reaction is self-limiting in thatthe reaction terminates once the initially adsorbed layer is consumed byreaction with the second reagent. Excess second reagent is purged and/orpumped away. The aforementioned steps constitute one deposition or ALD“cycle.” The process is repeated to form the next layer, with the numberof cycles determining the total deposited film thickness.

Returning to FIG. 13, the process 1300 begins with depositing a bottomelectrode on a substrate in operation 1302. The bottom electrode (e.g.the electrode 206) may be one of the electrode materials describedabove; however, in one embodiment, the bottom electrode is a polysiliconelectrode that may form a silicon dioxide interface layer 214 during thedeposition of the switching layer 210. In other embodiments, the bottomelectrode is a metal electrode that can also oxidize during thedeposition of the switching layer 210.

In operation 1304, a thin PVD metal oxide layer is optionally depositedon the substrate. The thin PVD metal oxide layer can be used toeliminate the interface layer 214 since the PVD deposition process hasbeen shown to not promote the growth of the interface layer 214 (seee.g. FIG. 5). Once the thin (e.g. <10 Å) PVD metal oxide layer isdeposited, the ALD process in operation 1306 can be performed.

In operation 1306, a switching layer 210 is deposited using ALD. Theoperation 1304 includes several component operations 1308-1320 thatdescribe several cycles of the ALD process. Some of these operations areoptional, or may be completed in a different order.

In operation 1308, the deposition temperature of the ALD process isoptionally lowered. The deposition temperature may be lowered bylowering the temperature of a heated substrate pedestal (i.e. thepedestal temperature), for example. In some examples, the depositiontemperature or pedestal temperature may be 250° C. or less, 200° C. orless, 175° C. or less, etc. The reduced temperature leads to incompleteALD reactions, leaving unreacted precursor ligands in the switchinglayer 210, increasing the amount of defects in the switching layer 210.Additionally, the reduced deposition temperature can reduce or eliminatethe interface layer 214 by reducing the rate of thermal oxidation. Forexample, when using a silicon electrode 206, reducing the ALD depositiontemperature to below 200° C. may substantially reduce any interfacelayer 214.

In operation 1310, the precursor source is maintained at a desiredpressure. The desired pressure can be achieved by controlling thetemperature of the precursor source. The precursor source may beexternal to the ALD deposition chamber, and may therefore be maintainedat a temperature different than the temperature of the depositionchamber. The desired temperature and pressure depends on the precursorused. For example, when using tetrakis(dimethlyamino)hafnium (TDMAH) todeposit hafnium oxide, the precursor source can be maintained at 30-100°C., or 40-50° C. In some embodiments, the temperature of the precursorsource can be increased to increase the partial pressure of theprecursor, which can also create a more metal-rich switching layer byincreasing the amount of pressure in the chamber. In operation 1212, theprecursor is introduced to the substrate including the bottom electrodeto begin the ALD process.

Operations 1314 and 1316 describe the treatment of the oxygen sourceused to form the metal oxide. Depending on the characteristics of thememory element 102, either or both of operations 1314 and 1316 can beused to control the thickness of the interface layer 214. The oxygensource can be ozone, oxygen, water vapor, isopropyl alcohol (IPA),ethanol or another alcohol, or other ALD oxygen sources.

In operation 1314, the oxygen source is maintained at a lower vaporpressure than is typical to create a switching layer 210 having lessoxygen. The vapor pressure can be reduced by reducing the temperature ofthe oxygen source. The temperature at which the oxygen source ismaintained will differ depending on the oxygen source and the metaloxide to be deposited. For example, ozone and oxygen tend to be moreoxidizing (i.e. more quickly create a layer having more oxygen), whilewater vapor is less oxidizing, and IPA and ethanol are less oxidizingstill. The lowered temperature reduces the vapor pressure of the oxygensource, which controls the amount of the oxygen source that isintroduced into the deposition chamber. Restricting the amount of theoxygen source in the chamber still allows the film to be self-limiting,while reducing the amount of oxygen in the film, as some of theprecursor ligands will be unbound. The oxygen-deficient film will thenhave oxygen vacancies, which are defects that can be used to control theswitching of the memory element 102.

To deposit a metal-rich hafnium oxide switching layer 210, for example,water vapor can be used as the oxygen source, and the water vapor sourcecan be held at a reduced temperature such as 0 to 10° C. The reducedtemperature reduces the vapor pressure of the oxygen source, effectivelyreducing the amount of oxygen introduced into the chamber and in theresulting film. Hafnium oxide films formed using this technique canresult in elemental compositions of HfO_(1.2) to HfO_(1.9), orHfO_(1.7). Generally, oxygen concentrations can be reduced to 60-95% ofstoichiometric compositions (i.e. the amount of oxygen is between 60 and95% of a stoichiometric metal oxide, e.g. HfO_(1.2) to HfO_(1.9)). IPAor ethanol can be used to provide oxygen, but at the same temperaturewill provide less oxygen than water vapor or the other oxygen sourcesdescribed above. IPA or ethanol may therefore be able to depositmetal-rich films using a room temperature source, although a similartemperature reduction can also be used with IPA and ethanol to reducethe amount of oxygen in the switching layer 210.

In operation 1318, the oxygen source is introduced to the substrate tocreate an ALD layer of metal oxide. A single ALD cycle may deposit afilm having a thickness of 0.5 Å, for example, and multiple cycles aretypically needed to build a switching layer 210 of the desiredthickness. In operation 1320, if more cycles are needed, the process1300 returns to operation 1308. If no more cycles are needed, theprocess 1300 continues to operation 1322.

In operation 1322, a coupling layer is deposited. The coupling layer 212can be a thin layer, for example less than 25 percent the thickness ofthe switching layer. The coupling layer 212 can be deposited using anydeposition method, such as ALD, PVD, etc. In operation 1324, the topelectrode (e.g. the electrode 208) is deposited.

In operation 1326, the memory element is annealed. The annealing canremove unreacted precursor ligands that may exist in the film because ofthe low deposition temperature of the ALD process. In one example, theelement is annealed using a hydrogen/argon mixture (e.g. 2-10% hydrogen,90-98% argon), although other anneals such as vacuum anneals, oxidizinganneals, etc. can be used.

Returning to FIG. 14, the process 1400 describes the formation of abottom electrode 206 for use in the memory elements 102. The process1400 describes the deposition of a silicide electrode that can be usedto remove the interface layer 214 if so desired. In some embodiments, asilicide electrode does not form an interface layer 214 during thedeposition of the switching layer 210. The process 1400 can in someembodiments, be used in conjunction with the process 1300. For example,the operations 1402-1412 of the process 1400 can be substituted into theoperation 1302 of the process 1300.

In operation 1402, a bottom electrode (e.g. the electrode 206) isdeposited on a substrate. The bottom electrode is a metal silicide, forexample a titanium, cobalt, nickel, palladium, or platinum silicide,that is deposited according to the operations 1404-1412.

In operation 1404, silicon is deposited on the substrate. In operation1406, a metal such as titanium, cobalt, nickel, molybdenum, palladium,or platinum is deposited on the silicon. In operation 1408 a thermaltreatment is performed to form the silicide layer by interdiffusing thesilicon into the metal. In operation 1410, any unreacted metal isstripped from the electrode, and in operation 1412, the electrode can beoptionally annealed to lower the resistivity of the electrode.

After the silicide electrode is deposited, in operation 1414, aswitching layer is deposited on the electrode (for example usingtechniques described in the process 1300), and in operation 1416 a topelectrode (e.g. the electrode 208) is deposited over the switchinglayer. The silicide electrode resists the formation of oxide layers, andtherefore does not form the interface layer 214. Although in someembodiments it may be desirable to retain an interface layer 214, inothers it is more desirable to eliminate the interface layer 214, andthe process 1400 is an alternative technique for doing so.

VI. Representative Data

A. Switching Characteristics

Table 3 contains various switching metrics for memory elements formedusing embodiments described herein, and other memory elements as acomparison:

-   -   HfO_(x)/TiO₂ refers to a memory element including an n-type        polysilicon electrode 206, a 50 Å thick hafnium oxide switching        layer 210 deposited at 250° C., an 8 Å titanium oxide coupling        layer 212 deposited at 250° C., and a titanium nitride electrode        208.    -   HfO_(x)/Al₂O₃ refers to a memory element including an n-type        polysilicon electrode 206, a 50 Å thick hafnium oxide switching        layer 210 deposited at 250° C., an 8 Å aluminum oxide coupling        layer 212 deposited at 250° C., and a titanium nitride electrode        208.

HfO_(x)/ZrO₂ refers to a memory element including an n-type polysiliconelectrode 206, a 50 Å thick hafnium oxide switching layer 210 depositedat 250° C., an 8 Å zirconium oxide coupling layer 212 deposited at 250°C., and a titanium nitride electrode 208.

-   -   HfAl_(x)O_(y) refers to a memory element including an n-type        polysilicon electrode 206, a 58 Å aluminum-doped hafnium oxide        switching layer 210 deposited at 250° C., and a titanium nitride        electrode 208.

TABLE 3 V_(FORM) V_(RESET) V_(SET) Yield HfO_(x)/TiO₂ −6 V to −9 V 6-7 V−3 V to −4 V 40-60% HfO_(x)/Al₂O₃ −6 V to −8 V 4-6 V −3 V to −4 V 50-70%HfO_(x)/ZrO₂ −5 V to −8 V 6-7 V −3 V to −4 V 60-80% HfAl_(x)O_(y) >|−8|V 6-7 V −4 V to −5 V 20-40%

All data are for bipolar switching, and yield refers to the percentagein a given sample of memory elements that reliably switch. As can beseen, the higher bandgap coupling layers in the HfO_(x)/Al₂O₃ andHfO_(x)/ZrO₂ memory elements show improved forming or reset voltages andimproved cycling yields.

The HfAl_(x)O_(y) and the HfO_(x)/Al₂O₃ memory elements have the samethickness and the same material components. However, the HfAl_(x)O_(y)memory element is aluminum-doped, and has the aluminum dispersedthroughout the hafnium oxide layer, while the HfO_(x)/Al₂O₃ memoryelement has a bulk hafnium oxide layer and a small aluminum oxidecoupling layer. The switching characteristics for the HfO_(x)/Al₂O₃ arebetter, suggesting that the improved switching may be due to defectsformed at the interface between the coupling layer 212 and the switchinglayer 210.

B. Interface Layer

Techniques described in the process 1300 were used to deposit a memoryelement 102 that substantially eliminated the interface layer 214.Aluminum oxide was deposited using trimethylaluminum and water vapor.The amount of water vapor in the gas phase was restricted by loweringthe temperature of the water vapor source to 1-5°. Using this technique,the thickness of the interface layer 214 was reduced from 1.1 nm (whenthe water source was held at room temperature) to approximately zero. Insome embodiments, elimination of the interface layer 214 may reduceforming voltage.

VII. Representative Embodiments

In accordance with an embodiment, a resistive-switching memory elementis provided that includes a first electrode and a second electrode, aswitching layer between the first electrode and the second electrodecomprising hafnium oxide and having a first thickness, and a couplinglayer between the switching layer and the second electrode, the couplinglayer comprising a material selected from the group consisting ofaluminum oxide and zirconium oxide, the coupling layer having a secondthickness that is less than 25 percent of the first thickness.

In accordance with a further embodiment, the first electrode of thememory element is doped silicon and the memory element is configured toreceive a negative reset voltage relative to a common electricalreference and a positive set voltage relative to the common electricalreference at the second electrode.

In accordance with a further embodiment, the first electrode of thememory element comprises a first material and the second electrodecomprises a second material, and the first material is different fromthe second material.

In accordance with a further embodiment, the first material of the firstelectrode is doped silicon and the second material of the secondelectrode is titanium nitride.

In accordance with a further embodiment, the first thickness of thememory element is between 20 and 100 angstroms.

In accordance with a further embodiment, the switching layer of thememory element comprises a hafnium oxide material having an elementalcomposition of between HfO_(1.2) and HfO_(1.7).

In accordance with a further embodiment, the first material of the firstelectrode is n-type polysilicon.

In accordance with a further embodiment, at least one of the firstelectrode and the second electrode of the memory element has a same mostcommon oxidation state as the switching layer.

In accordance with a further embodiment, the memory element furtherincludes an interface layer between the first electrode and theswitching layer, the interface layer having a thickness less than 10 Å.

In accordance with a further embodiment, the interface layer of thememory element comprises silicon oxide.

In accordance with a further embodiment, a work function of the secondelectrode of the memory element is greater than a work function of thefirst electrode, and wherein the first electrode is configured toreceive a forming voltage pulse having a negative voltage relative to acommon electrical reference.

In accordance with another embodiment, a resistive-switching memoryelement is provided, including a first electrode and a second electrode,a switching layer between the first electrode and the second electrode,the switching layer comprising a first metal oxide having a firstbandgap greater than 4 electron volts (eV), the switching layer having afirst thickness, and a coupling layer between the switching layer andthe second electrode, the coupling layer comprising a second metal oxidehaving a second bandgap greater than or equal to the first bandgap, thecoupling layer having a second thickness that is less than 25 percent ofthe first thickness.

In accordance with a further embodiment, the first metal oxide of thememory element has an oxygen concentration that is between 60 and 95% ofstoichiometric.

In accordance with a further embodiment, the first electrode of thememory element is selected from the group consisting of doped siliconand titanium nitride, and the second electrode is selected from thegroup consisting of molybdenum nitride, molybdenum oxide, titaniumnitride, tungsten, tantalum nitride, molybdenum nitride, molybdenumoxide, platinum, ruthenium, nickel, iridium, iridium oxide, andruthenium oxide.

In accordance with a further embodiment, the first thickness of theswitching layer is between 20 and 100 Å.

In accordance with a further embodiment, a first metal of the firstmetal oxide has a first most common oxidation state that is differentfrom a second most common oxidation state of the second metal of thesecond metal oxide.

In accordance with a further embodiment, a first metal of the firstmetal oxide and a second metal of the second metal oxide have a samemost common oxidation state.

In accordance with a further embodiment, a second metal of the secondmetal oxide has a second most common oxidation state that is than lessthan or equal to a first most common oxidation state of a first metal ofthe first metal oxide.

In accordance with a further embodiment, the first metal oxide ishafnium oxide and the second metal oxide is selected from the groupconsisting of zirconium oxide and aluminum oxide.

In accordance with a further embodiment, the first metal oxide isselected from the group consisting of hafnium oxide, tantalum oxide,aluminum oxide, zirconium oxide, and yttrium oxide, and the second metaloxide is selected from the group consisting of zirconium oxide andaluminum oxide.

In accordance with a further embodiment, the first electrode comprisesdoped silicon and further comprising an interface layer between thefirst electrode and the switching layer comprising silicon oxide andhaving a thickness of less than 10 Å.

In accordance with a further embodiment, the first electrode comprises asilicide chosen from the group consisting of titanium silicide, cobaltsilicide, nickel silicide, palladium silicide, and platinum silicide.

In accordance with a further embodiment, the memory element is part of athree-dimensional memory array.

In accordance with another embodiment, a method for forming aresistive-switching memory element is provided, including depositing afirst electrode on a substrate, depositing a switching layer comprisinga metal oxide over the first electrode using atomic layer deposition(ALD), the depositing the switching layer further comprising maintaininga precursor at greater than 40 degrees Celsius, introducing theprecursor to the substrate, maintaining an oxygen source at less than 10degrees Celsius, introducing the oxygen source to substrate, anddepositing a second electrode over the switching layer.

In accordance with a further embodiment, the oxygen source is at leastone of water vapor, isopropyl alcohol (IPA), and ethanol.

In accordance with a further embodiment, the metal oxide is chosen fromthe group consisting of hafnium oxide, tantalum oxide, aluminum oxide,yttrium oxide, and zirconium oxide.

In accordance with a further embodiment, the metal oxide has an oxygenconcentration that is between 60 and 95 percent of stoichiometric.

In accordance with a further embodiment, the metal oxide is hafniumoxide and has an elemental composition that is between HfO_(1.2) andHfO_(1.7).

In accordance with a further embodiment, a deposition temperature forthe ALD is less than 250 degrees Celsius.

In accordance with a further embodiment, a deposition temperature forthe ALD is approximately 200 degrees Celsius.

In accordance with a further embodiment, the method includes annealingthe memory element after depositing the second electrode.

In accordance with a further embodiment, the method further includesdepositing a physical vapor deposition (PVD) layer over the firstelectrode using physical vapor deposition, wherein the switching layeris deposited over the PVD layer, and wherein the PVD layer comprises asame material as the switching layer.

In accordance with a further embodiment, the method further includesdepositing a coupling layer over the switching layer, the coupling layerhaving a thickness that is less than 25 percent of a thickness of theswitching layer.

In accordance with a further embodiment, a bandgap of the coupling layeris greater than a bandgap of the switching layer.

In accordance with a further embodiment, the coupling layer is selectedfrom the group consisting of aluminum oxide and zirconium oxide.

In accordance with another embodiment, a method for forming aresistive-switching memory element including depositing a firstelectrode on a substrate, depositing a switching layer over the firstelectrode, the switching layer having a first thickness and a firstbandgap that is greater than 4 electron volts (eV), depositing acoupling layer over the switching layer, the coupling layer having asecond thickness that is less than 25 percent of the first thickness anda second bandgap that is greater than or equal to the first bandgap, anddepositing a second electrode over the coupling layer.

In accordance with a further embodiment, the first electrode is dopedsilicon and depositing a switching layer comprises forming an interfacelayer comprising silicon oxide between the first electrode and theswitching layer, the interface layer having a thickness of less than 10Å.

In accordance with a further embodiment, the switching layer is chosenfrom the group consisting of hafnium oxide, tantalum oxide, aluminumoxide, yttrium oxide, and zirconium oxide.

In accordance with a further embodiment, the coupling layer is chosenfrom the group consisting of zirconium oxide and aluminum oxide.

In accordance with a further embodiment, depositing the switching layercomprises using atomic layer deposition (ALD), including maintaining aprecursor at greater than 40 degrees Celsius, introducing the precursorto the substrate, maintaining an oxygen source at less than 10 degreesCelsius, and introducing the oxygen source to the substrate.

In accordance with a further embodiment, depositing the switching layercomprises depositing a metal oxide having an oxygen concentration thatis between 60 and 95 percent of stoichiometric.

In accordance with a further embodiment, the method includes annealingthe memory element.

In accordance with a further embodiment, the oxygen source is at leastone of water vapor, isopropyl alcohol, and ethanol.

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

What is claimed:
 1. A resistive-switching memory element comprising: afirst electrode and a second electrode; a switching layer between thefirst electrode and the second electrode comprising hafnium oxide havingan elemental composition of between HfO_(1.2) and HfO_(1.7) and having afirst thickness; and a coupling layer between the switching layer andthe second electrode, the coupling layer comprising a material selectedfrom the group consisting of aluminum oxide and zirconium oxide, thecoupling layer having a second thickness that is less than 25 percent ofthe first thickness.
 2. The memory element of claim 1, wherein the firstelectrode is doped silicon and the memory element is configured toreceive a negative reset voltage relative to a common electricalreference and a positive set voltage relative to the common electricalreference at the second electrode.
 3. The memory element of claim 1,wherein the first thickness is between 20 and 100 angstroms.
 4. Thememory element of claim 1, wherein the first electrode comprises n-typepolysilicon.
 5. The memory element of claim 1, wherein at least one ofthe first electrode and the second electrode has a same most commonoxidation state as the switching layer.
 6. The memory element of claim1, further comprising an interface layer between the first electrode andthe switching layer, the interface layer having a thickness less than 10Å.
 7. The memory element of claim 3, wherein a work function of thesecond electrode is greater than a work function of the first electrode,and wherein the first electrode is configured to receive a formingvoltage pulse having a negative voltage relative to a common electricalreference.
 8. A resistive-switching memory element comprising: a firstelectrode and a second electrode; a switching layer between the firstelectrode and the second electrode, the switching layer comprising afirst metal oxide, the first metal oxide comprising hafnium oxide havingan elemental composition of between HfO_(1.2) and HfO_(1.7), theswitching layer having a first thickness; and a coupling layer betweenthe switching layer and the second electrode, the coupling layercomprising a second metal oxide having a bandgap greater than or equalto a bandgap of the first metal oxide, the coupling layer having asecond thickness that is less than 25 percent of the first thickness. 9.The memory element of claim 8, wherein the first metal oxide has anoxygen concentration that is between 60 and 95% of stoichiometric. 10.The memory element of claim 8, wherein: the first electrode is selectedfrom the group consisting of doped silicon and titanium nitride; and thesecond electrode is selected from the group consisting of molybdenumnitride, molybdenum oxide, titanium nitride, tungsten, tantalum nitride,molybdenum nitride, molybdenum oxide, platinum, ruthenium, nickel,iridium, iridium oxide, and ruthenium oxide.
 11. The memory element ofclaim 8, wherein a first metal of the first metal oxide has a first mostcommon oxidation state that is different from a second most commonoxidation state of the second metal of the second metal oxide.
 12. Thememory element of claim 8, wherein a first metal of the first metaloxide and a second metal of the second metal oxide have a same mostcommon oxidation state.
 13. The memory element of claim 8, wherein asecond metal of the second metal oxide has a second most commonoxidation state that is than less than or equal to a first most commonoxidation state of a first metal of the first metal oxide.
 14. Thememory element of claim 8, wherein: the first metal oxide is selectedfrom the group consisting of hafnium oxide, tantalum oxide, aluminumoxide, zirconium oxide, and yttrium oxide; and the second metal oxide isselected from the group consisting of zirconium oxide and aluminumoxide.
 15. The memory element of claim 8, wherein the first electrodecomprises doped silicon and further comprising an interface layerbetween the first electrode and the switching layer comprising siliconoxide and having a thickness of less than 10 Å.
 16. The memory elementof claim 8, wherein the first electrode comprises a silicide chosen fromthe group consisting of titanium silicide, cobalt silicide, nickelsilicide, palladium silicide, and platinum silicide.